I haven't changed any of my lightroom settings but for some reason the last few imports I have done, unless the SD card that the raw files are on is inserted, when I click on them in develop- I get an error message that says they are offline or missing. Does anybody have any suggestions for how to fix this or where the files could be? I want to be able to erase the memory cards for future shoots but want to have the option to re-edit the photos later on.

You have somehow accidentally imported the photos on your SD card using the ADD option. This means that Lightroom will look for the photos on the SD card only. What you need to do is copy the photos to your hard disk using your operating system, and then eject the SD card and tell Lightroom where the photos are now located on your hard disk. To tell Lightroom where the photos are now located, please follow these instructions: Adobe Lightroom - Find moved or missing files and folders

...Does anybody have any suggestions for how to fix this or where the files could be?...

Obviously the files are on your SD Card. You imported them using the "Add" option rather than "Copy". I suggest you copy them from the SD card to the folder where you originally intended to put them using your computer OS and then reconnect Lightroom to them from within Lightroom. After they are reconnected and after you made backups of the photo files...(not the catalog but the photo files themselves)...only then can you safely reformat your SD card in camera.

If you have edited these images using Lightroom, and don't want to have to start over, then you're going to have to move them using Lightroom and move them to your hard drive. You'll have to be careful because it's easy to lose images in the process. But I think it would probably be the best approach in this particular situation.